The human immunodeficiency viruses, HIV-1 and HIV-2, are members of an extensive but poorly characterized family of primate lentiviruses that appear to have their origins in African primates. Each of the human viruses presumably arose relatively recently following cross-species transmission from a naturally-infected primate to humans. In the case of HIV-2, the precursor appears to be SIVsm from sooty mangabey monkeys (Cercebus atys) whereas the origins of HIV-1 are less clear. The goals of this project are twofold. The first is to molecularly characterize novel SIV isolates from wild-caught African monkeys more extensively with a particular emphasis on those isolated from members of the Cercopithecus genus. The underlying hypothesis of this project is that the primate lentiviruses originated and co-evolved within monkeys of the Cercopithecus genus and that examples of SIV strains in other monkeys are evidence of more recent cross-species transmission. We initiated these studies by characterizing SIV isolates from three of the species of African green monkeys (vervets, grivets and tantalus), then SIV from Sykes monkeys (C. albogularis) and most recently SIV from lhoest monkeys (C. lhoesti). SIVlhoest is a new member of the SIVmnd lineage, initially named for mandrill (Mandrillus sphinx). The geographic distance in the habitats of lhoest monkeys and mandrills rule out the possibility that either virus resulted from cross-species transmission between mandrills and lhoest monkeys. The distant phylogenetic relationship between these monkeys also is inconsistent with co-evolution in their respective host species. Thus we proposed that SIVmnd might be the result of cross-species transmission of SIV from a West African relative of the lhoest monkey, either suntailed (C. solatus) or preussis (C. presussi) monkeys. To evaluate this hypothesis we characterized SIVsun isolated from a suntailed monkey. SIVsun clusters phylogenetically with SIVlhoest establishing the species-specific evolution of this lineage. However, SIVmnd was no more closely related to SIVsun than SIVlhoest ruling out this virus as the immediate precursor for SIVmnd. Studies in the Peeters lab have recently revealed that mandrills are infected with two distinct types of SIVs, one being the previously characterized virus now named SIVmnd-1. The other appears to be a recombinant between SIVmnd-1 and SIV from drills (SIVdrl). Both SIVmnd-2 and SIVdrl have regions of homology with SIV from redcapped mangabeys (SIVrcm) but neither SIVrcm nor SIVdrl have been fully characterized. Recently we cloned and sequenced the entire genome of SIVrcm isolated from a wild caught red capped mangabey from Nigeria and demonstrated that this virus is a complex recombinant and are in the process of characterizing SIVdrl. A secondary but equally important goal of this project is to study the mechanisms underlying the apparent lack of pathogenicity of SIV for their natural host species with emphasis on SIVagm from vervet monkeys. Both SIVsm from sooty mangabeys and SIVagm from African green monkeys are capable of inducing AIDS in macaques but are not virulent for their natural host. We also evaluated the pathogenicity of SIVlhoest and SIVsun in macaques and observed robust primary viremia, lymphadenopathy and declining CD4 lymphocyte counts consistent with progression to AIDS. Therefore, SIVlhoest and SIVsun are also capable of inducing AIDS in macaques. To more extensively study the underlying mechanisms for differential virulence of SIV, we derived the molecularly cloned, pathogenic SIVagm9063-2 that induces AIDS in pigtailed macaques and Asian species but results in asymptomatic infection of African green monkeys. Most recently we have derived primary isolates of SIV from naturally-infected vervets that will be used in subsequent animals studies. A quantitative realtime RT-PCR assay for measuring viral RNA in plasma samples was developed as well as assays for identifying target cells of SIV in such animals by confocal microscopy. Natural and experimental infection of vervets with SIVagm was evaluated by in situ hybridization, plasma viral load assays and limiting dilution coculture to identify the numbers and distribution of infected cells in tissues. Virus was found in lymphoid tissues and the gastrointestinal tract and in the lung macrophages of one animal. Plasma viral load varied widely from undetectable (<1000 copies/ml) to 800,000 copies/ml. This wide range in viral load in naturally-infected AGM was unexpected since naturally-infected sooty mangabey monkeys exhibit very high levels of plasma viremia in contrast. We have initiated a study to evaluate the viral kinetics of a primary SIVagm isolate in AGM and pigtailed monkeys. These studies will be the basis for proceeding on to intervention studies with chemical immunosuppression or depletion of CD8 T cells with specific monoclonal antibodies. Preliminary results show that the primary plasma viral load in inoculated AGM ranges from undetectable to 100,000 copies/ml, whereas viremia in the pigtailed macaques was significantly higher (10 to 100 million copies/ml). Pigtailed macaques exhibited early signs of SIV-induced disease (lymphdeopathy, and early CD4 depletion) whereas no clinical or pathologic changes have been observed in the AGM.